Mogas: what happens to EGTs?

"Jim Weir" wrote in message
news
" jls"
shared these priceless pearls of wisdom:

-
-And if the burn is cooler when 100LL (rather than 80) is run through the
-carb, why is it the engine must have special 100LL exhaust valves, which
are
-designed with high-temperature alloys?

The alloy of a valve in a low-compression Continental engine is precisely
the
same alloy as the 100LL exhaust valve. The 100LL valve has had some
machine
work and clearance work (mainly in the stem area and in the angle of the
valve
seat area) done to make it compatible with the excess 100LL lead that jams
valves.

I stand corrected, then. Thanks for your lecture and I note that you
rubbed it in a little too. OK, all in a day's fun. I can take it.
Sniffle I had read that the 100 octane valve was a different alloy but
always wondered if it were so. It sure is a comparatively expensive little
bugger.

-I don't know the answer but I tell you what I suspect. I suspect the
egt
-of 100LL will be higher.

You suspect wrong. I have made this exact same measurement on several
low-compression Continentals before I started using mogas (primarily the
O-300D,
but a couple of more engines to a lesser extent. Mogas burns hotter
according
to the EGT.

Thanks. I will find out soon with respect to my own O-300 which will have
EGT sensors and will compare results. Btw I enjoyed the poke you took at
the poor guy who had chickens in his sparkplugs. He was right gentlemanly
about the poke too.


-Higher octane gasoline like 100LL, furthermore, is a little less
volatile
-than mogas and burns more slowly because of the higher RON numbers, so
that
-when the exhaust valve opens combustion is less complete than with the
same
-charge of mogas -- in the low-compression O-300. So if you're running
-100LL in an O-300-powered 172, why do you need 100-octane exhaust
valves,
-whose faces are something like inconel or another exotic alloy designed
for
-temperatures much higher than the stock exhaust valves?

You predicate your entire argument on the fallacy of "exotic alloy"
valves, when
in fact, the only difference is in the settings of the lathe that cuts
them.

Jim

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com